Changing User IDs as well as Group IDs is the standard Debian way. Keep in mind that you might need to change the existing files, too. You can find both informations here for example: cyberciti.biz/faq/linux-change…-gid-for-all-owned-files/

My plans for a small and compact NAS are exactly the same. HC2 and a big single HDD. Everything important is synced to another remote cloud solution in the meantime. So it does not really matter if the HDD crashes in the future. However - I don´t like to spend so much money on a single HDD... 8TB would be enough in my case.

Don´t you use iCloud? Thats for almost everything except Photos. For Photos you can use for example: photosync-app.com/ Quote from Photosync-App: “Securely backup photos & videos to any NAS, mobile storage device or personal cloud over SMB, (S)FTP or WebDav. PhotoSync works flawlessly with Synology, Western Digital, QNAP, Netgear, Seagate, Toshiba, SanDisk, Buffalo… ” Where are your PDFs as well as your other important docs saved? Or what app do you use to open/modify them?

Quote from newbieHC1HC2: “2x iPads both have tons of data that needs backup. I gotta figure out APPLE OS iOS V10.3.3 ( NEWEST IT WILL SUPPORT) I DONT KNOW iOs AND HOW TO SETUP NAS. ” What exactly do you mean with "tons of data"? Depending on what you want to save exactly I can give you some information. (Contacts, Calendar? Or do you have it at apple`s icloud? Photos?...?)

Data disks will not get harmed if you change the OS drive. (Please unplug them while installing on your new OS drive!) IMHO it is not worth making a backup from the OS drive and fiddling it onto a new drive. I´m always installing OMV freshly onto the new drive, mount the data drives and configure everything from scratch. Takes about 1 hour - done. However - a backup of your data (depending on how important it is) is always a good idea.

OMV does just sit on top of Debian. Therefore if Debian releases updates you still receive them. And guess what, those are such updates So, OMV itself would appear in that list. Just out of curiosity - why don´t you want to move?